A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned.
Lithography is widely recognized as one of the key steps in the manufacture of ICs and other devices and/or structures. However, as the dimensions of features made using lithography become smaller, lithography is becoming a more critical factor for enabling miniature IC or other devices and/or structures to be manufactured.
A theoretical estimate of the limits of pattern printing can be given by the Rayleigh criterion for resolution as shown in equation (1):
                    CD        =                              k            1                    ⋆                      λ                          NA              PS                                                          (        1        )            
where λ is the wavelength of the radiation used, NAPS is the numerical aperture of the projection system used to print the pattern, k1 is a process dependent adjustment factor, also called the Rayleigh constant, and CD is the feature size (or critical dimension) of the printed feature. It follows from equation (1) that reduction of the minimum printable size of features can be obtained in three ways: by shortening the exposure wavelength λ, by increasing the numerical aperture NAPS or by decreasing the value of k1.
In order to shorten the exposure wavelength and, thus, reduce the minimum printable size, it has been proposed to use a source to provide extreme ultraviolet (EUV) radiation (sometimes termed soft x-ray). An EUV radiation source is configured to output a radiation wavelength of about 13 nm. Thus, an EUV radiation source may constitute a significant step toward achieving small features printing. Possible EUV radiation sources include, for example, a laser-produced plasma source, a discharge plasma source, or synchrotron radiation from electron storage rings.
The transmission of optical materials and of fluids generally deteriorates with decreasing wavelength. Hence EUV radiation may be substantially absorbed when traversing optical materials or fluids. A lithographic apparatus which uses EUV radiation to apply a pattern to a substrate may therefore operate under vacuum so that only a within tolerance absorption of EUV radiation may be present due to a residual, small amount of gas. A vacuum environment may be provided along a part or all of a path of the radiation beam through the lithographic apparatus with the aid of one or more vacuum walls and vacuum pumps. Since EUV radiation is readily absorbed by optical materials of refractive optical elements, there is a general tendency (i.e. a prejudice) to ensure that optical elements used to condition, pattern, direct or re-direct an EUV radiation beam in a lithographic apparatus are embodied as reflective elements, and not as refractive, transmissive elements.
In order to effectively reflect EUV radiation, reflective optical elements have to be carefully designed and constructed. Many optical elements used for reflecting BUY radiation comprise one or more layers, and are often referred to as being or comprising multilayer structures. A multilayer structure will typically reflect 65-70% of EUV radiation that is incident upon the structure. It will therefore be appreciated that the intensity of an EUV radiation beam reflected off such a multilayer structure will have its intensity reduced by 30-35% for each reflection. In a reflective EUV radiation lithographic apparatus, a radiation beam may be reflected on numerous occasions, and off numerous different optical elements, severely reducing the intensity of the radiation beam that is available to apply a pattern to a substrate. Such a reduction in intensity can have an adverse affect on the throughput of an EUV radiation lithographic apparatus. Furthermore, the need to reflect the radiation beam along a folded optical path in the lithographic apparatus may lead to an increased complexity in the design of the lithographic apparatus, and/or, for example, an increase in size or the cost of the apparatus.